1. Field of the Invention
The present invention relates to a deposition method by plasma CVD. More particularly, the present invention relates to a silicon carbide film which acts as an etch stop film used for copper wiring.
2. Description of the Related Art
Conventionally, in LSI devices including CPUs, memories, system LSIs, aluminum alloy has been used for metal interconnects. To insulate aluminum interconnects, a silicon oxide film is used; for some LSI devices, a fluorine-containing silicon oxide film is used.
In recent years, copper which has lower electrical resistivity than aluminum has been adopted as a material for metal wiring to speed up LSI devices' performance, and a carbon-containing silicon oxide film having a low dielectric constant has started to be adopted as an interlayer insulating film to reduce interconnect capacity which causes signal delay. In an LSI device having this copper wiring, an etch stop film is used to form trenches or vias for the copper wiring in a barrier layer of a carbon-containing silicon oxide film. In the past, a silicon nitride film, whose dry etching rate is slower as compared with the carbon-containing silicon oxide film and which has a function to prevent copper diffusion, has been used as an etch stop film. Because a dielectric constant of the silicon nitride film, however, is approximately 7, which is high, a problem that the silicon nitride film increases interconnect capacity on the whole occurs even if the carbon-containing silicon oxide film is applied.
To solve this problem, a technology using a silicon carbide film as an etch stop film was developed as described in, for example, U.S. Pat. No. 5,800,878.
A dielectric constant of a silicon carbide film is approximately 5 and is applied to LSI devices using copper wiring by combining with a carbon-containing silicon oxide film having a dielectric constant of approximately 3.
Typically, what are called silicon carbide films have several different compositions. One of them is a silicon carbide film comprising Si, C, and H. This film has a property of absorbing oxygen or moisture easily and has a shortcoming that its film stress or dielectric constant is changed if it is left in the atmosphere. Additionally, the film has high leakage current and provides poor electrical isolation. To solve these problems, a technology to prevent moisture or oxygen from intruding from a film surface by processing the film surface by an inert plasma gas was developed as described in, for example, U.S. Patent Publication No. 2002-0054962. The method to minimize the oxidation of carbon containing materials, such as silicon carbide, with an inert gas plasma such as helium (He), Argon (Ar) is also published in JP laid-open patent 2001/0060584.
This method, however, only improves the quality of the very surface of the film; it does not improve internal characteristics of the SiCH film; the film still has high leakage current and provides poor electrical isolation. The dielectric constant of this film is approximately 4.5 to 5; hence, application of the film to current LSI devices aiming to perform faster is difficult.
As silicon carbide films having different compositions, a silicon carbide film comprising Si, C, N, and H, a silicon carbide film comprising Si, C, O, and H, and others were disclosed, for example, in U.S. Patent Publication No. 2001-0030369, U.S. Patent Publication No. 2002-0027286, U.S. Patent Publication No. 2001-0051445, and U.S. Patent Publication No. 2001-0031563.
These silicon carbide films have lower leakage current and provide better electrical isolation as compared with the above-mentioned SiCH film. Particularly, a SiCOH film can attain a low dielectric constant of approximately 4.2 depending on an oxygen ratio. The silicon carbide films have a problem, however, that due to their chemical properties similar to a carbon-containing silicon oxide film, their etching rate difference with the carbon-containing silicon oxide film's becomes small (within 3 to 4 times), lowering their etch stop performance. Particularly, because the SiCOH film whose dielectric constant is lowered by increasing its oxygen content has a chemical composition similar to the carbon-containing silicon oxide film which is used as an insulating film for the interconnects, it is etched at a rate close to an etching rate of the carbon-containing silicon oxide film, making difficult to form vias or trenches for copper wiring to be formed by Reactive Ion Etching (RIE) using C4F8+O2+Ar gas. Additionally, to speed up LSI devices' performance, attaining a low dielectric constant of 4 or lower is demanded for the etch stop film.